In thermally assisted optical/magnetic data storage, information bits are recorded on a layer of a storage medium at elevated temperatures, and the heated area in the storage medium determines the data bit dimension. In one approach, an electromagnetic wave in the form of light is used to heat the storage medium. To achieve high areal data density, it is preferred to have a high light throughput to an optical spot well below the diffraction limit to heat the storage layer of the medium. Some prior systems have confined the light to a small spot but did not deliver a reasonable amount of optical power to the storage medium.
Heat assisted magnetic recording (HAMR) generally refers to the concept of locally heating a recording medium to reduce the coercivity of the recording medium so that the applied magnetic writing field can more easily direct the magnetization of the recording medium during the temporary magnetic softening of the recording medium caused by the heat source. Heat assisted magnetic recording allows for the use of small grain media, which is desirable for recording at increased areal densities, with a larger magnetic anisotropy at room temperature to assure sufficient thermal stability. Heat assisted magnetic recording can be applied to any type of magnetic storage media, including tilted media, longitudinal media, perpendicular media and patterned media.
Heat assisted magnetic recording requires an efficient technique for delivering large amounts of light power to the recording medium confined to spots of, for example, 50 nm or less. Areal density and bit aspect ratio are among the factors which determine this size. Based on previous studies, 1 Tb/in2 requires spots of 25 nm. A variety of transducer designs have been proposed and some have been experimentally tested. Among these are metal coated glass fibers and hollow pyramidal structures with metal walls. For all these approaches, confinement of the light depends on an aperture which is fabricated at the end of the structure and gives this kind of transducer the name “aperture probes.” Generally these devices suffer from very low light transmission rendering the devices useless for HAMR recording. For example, tapered and metallized optical fibers have demonstrated light confinement down to approximately 50 nm with a throughput efficiency of 10−6. Pyramidal probes made from anisotropic etching of Si wafers have been designed with throughput efficiencies of 10−4 for similar spot sizes. Although this is the state of the art, it is still about two orders of magnitude too small for HAMR.
Improvements in throughput efficiency have been achieved for these transducers by changing the taper angles, filling the hollow structures with high index materials, and trying to launch surface plasmons (SP) on integrated edges and corners of these tip-like structures. Although doing so does increase the throughput to some extent, the most promising SP approach is still very inefficient due to a lack of an efficient SP launching technique. In addition, all aperture probes suffer from a lower limit on spot size which is twice the skin depth of the metal film used to form the aperture. Even for aluminum, the metal with the smallest skin depth for visible light, this corresponds to a spot size of ˜20 nm.
Solid immersion lenses (SILs) and solid immersion mirrors (SIMs) have also been proposed for concentrating far field optical energy into small spots. The optical intensity is very high at the focus but the spot size is still determined by the diffraction limit which in turn depends on the refractive index of the material from which the SIL or SIM is made. The smallest spot size which can be achieved with all currently known transparent materials is ˜60 nm, which is too large for HAMR.
A metal pin can be used as a transducer to concentrate optical energy into arbitrarily small areal dimensions. In previously proposed designs that utilize a relatively long cylindrical metal pin located at a focal point, the pin supports a surface plasmon mode which propagates along the pin, and the width of the external electric field generated by the surface plasmon mode is proportional to the diameter of the pin.
There is a need for transducers that can provide a reduced spot size and increased throughput efficiencies.